There are many applications wherein light influencing displays are utilized to advantage. For example, light influencing displays find use in computer terminals, avionic information displays, digital watches, digital clocks, calculators, portable television receivers, and various forms of portable games.
Light influencing displays can be formed in many configurations using a number of different types of light influencing materials. By the term "light influencing material" is meant any material which emits light or can be used to selectively vary the intensity, phase, or polarization of light either being reflected from or transmitted through the material. Liquid crystal material is only one such material having these characteristics. Generally, each pixel includes a pair of electrodes which can be individually addressable and liquid crystal material between the electrodes. As is well known, when a voltage is applied across the electrodes which exceeds the voltage threshold of the liquid crystal material, the optical properties of the liquid crystal material between the electrodes can be changed to provide a light or dark display depending upon the type of material used and the desired mode of operation of the display.
Liquid crystal displays generally include a large number of pixels (picture elements) arranged in a matrix of rows and columns. Because of the large number of pixels in the matrix arrays, multiplexing is used to selectively address each pixel. To that end, each row of pixels in a common electrode plane are coupled together by row address lines and each column of pixels in the other common electrode plane are coupled together by column address lines. As a result, each pixel is located near a unique intersection of two address lines and is individually addressed by applying a voltage potential across its two intersecting lines. A passive matrix is formed when the pixel electrodes are directly coupled to the address lines. In such a matrix only the innate voltage threshold characteristic of the display material is relied upon to achieve selective actuation of only those pixels which are addressed with potentials greater than the threshold voltage. Thus, pixels can experience an increased voltage potential, because they are coupled to one of the address lines with an applied potential, but they will not be activated because the potential increase caused by the potential on one line is below the threshold voltage of the pixel. It is well known that the number of pixels which can be employed in liquid crystal displays using a passive matrix is limited by problems of pixel contrast and speed, which is dependent in part on the finite sharpness of the threshold voltage characteristics of the liquid crystal material.
To achieve high resolution, acceptable contrast and speed in displays having large numbers of pixels, displays using active matrices have been developed. Active matrix displays employ one or more isolation devices at each pixel to provide improved threshold voltage sharpness at each pixel and enhanced isolation from applied potentials between the pixels on the common address lines. A number of different types of two, three or four terminal isolation devices can be used to provide the required isolation. By the term "isolation device" is meant any device which enhances the ability for one pixel to be addressed (switched) without switching or adversely affecting other pixels sharing one or more common address lines. Such isolation devices can include threshold devices such as one or more diodes arranged in various configurations, M-I-M structures, etc., all of which provide a more precise voltage threshold than that provided by the light influencing material itself. A more precise voltage threshold means a smaller variance in the voltage required to switch the pixel from off to on. Other examples of isolation devices can include switching devices, such as thin film transistors.
Some two terminal isolation devices, such as diodes and some configurations of three terminal devices can be though of as single polarity devices, which can be turned on in only one direction or polarity. Three terminal devices, such as thin film transistors and other two terminal devices, such as diode rings, metal-insulator-metal (M-I-M) devices, and n.sup.+ -i-n.sup.+ and n.sup.+ -pi-n.sup.+ threshold isolation devices, can be thought of as dual polarity devices which can be turned on so as to conduct current in either one of two directions through the switch or device.
One problem in fabricating active matrix light influencing displays is yield. Virtually 100 percent of all of the isolation devices must be operational to obtain a usable display. Such extremely high yields can be difficult to achieve for large area displays, because the making of active matrix displays requires numerous process steps, and because a number such steps require extremely accurate photolithography over large dimensions, which is generally difficult to achieve.
Liquid crystal displays which can be manufactured with high yields and which utilize diodes for isolation devices are disclosed U.S. applications Ser. Nos. 573,004 abandoned, and 675,941, each entitled "Liquid Crystal Displays Operated By Amorphous Silicon Alloy Diodes", and filed in the names of Zvi Yaniv, Vincent D. Cannella, Gregory L. Hansell and Louis D. Swartz, on Jan. 23, 1984 and Dec. 3, 1984 respectively, which applications are incorporated herein by reference. As disclosed therein, the diodes can be formed with reduced precision photolithography and in fewer process steps than that required to form some of the prior isolation devices.
The displays disclosed in the aforementioned referenced pending U.S. applications and those generally known in the art most often must rely upon only on the charge retention due to the pixel capacitance resulting from the pair of electrodes and liquid crystal material to maintain a pixel in a desired optical condition. The total amount of charge which can be stored at a pixel location, relative to the overall conductance of the light influencing material (and any other leakage paths) available to discharge the stored charge, directly influences how successfully a desired voltage above some minimum threshold level can be held across the liquid crystal display material of a pixel so as to maintain the pixel in a given optical condition after the potentials are applied and during the time in which the other pixels of the display are being addressed. In other words, as total capacitance at the pixel is increased relative to the leakage paths available to discharge the pixel, the ratio between the peak voltage applied across the pixel to charge the pixel and the resultant RMS voltage across the pixel is decreased. Adding additional or auxiliary capacitance to increase the total amount of charge which may be stored is difficult because the added capacitance must be applied in parallel with the pixel capacitance across the electrodes which requires an electrical connection through the liquid crystal material. The addition of auxiliary capacitance is further complicated by the fact that the displays have addressing circuitry on both electrode planes.
An improved active matrix display having all of the addressing electronic circuitry, including isolation devices, on one substrate or pixel electrode plane of the display is disclosed in U.S. Pat. No. 4,589,733 entitled "Displays And Subassemblies Having Improved Pixel Electrodes", which issued on May 20, 1986 in the names of Zvi Yaniv, Yair Baron, Vincent D. Cannella and Gregory L. Hansell. This patent is hereby incorporated herein by reference. The displays disclosed there include a plurality of pixels, with each pixel including a first electrode including a pair of spaced apart side-by-side electrode portions on one plane and a second electrode spaced from and facing the first electrode portions on a second plane. Liquid crystal material is disposed between the first electrode portions and the second electrode. The second electrode is electrically insulated from all external circuit connections and from all other pixel electrodes. These displays exhibit decreased electronic complexity because all of the addressing lines are formed on the electrode plane of the first pixel electrode. In accordance with their preferred embodiments, the address lines are coupled to each first electrode portion by one or more isolation devices to provide a high degree of pixel isolation.
An improvement to the displays of the aforementioned U.S. Pat. No. 4,589,733 is disclosed in U.S. Pat. application Ser. No. 639,001 filed Aug. 8, 1984 U.S. Pat. No. 4,639,087, in the name of Vincent D. Cannella for "Displays And Subassemblies Having Optimized Capacitance", which application is also incorporated herein by reference. As disclosed therein, an auxiliary capacitance is provided to the pixels by the addition of a third or auxiliary capacitance electrode which is spaced from and facing the first electrode portions on the side of the first electrode opposite the liquid crystal display material. As a result of this structure, the third electrode of each pixel provides an auxiliary capacitance in parallel with the pixel capacitance. Practice has shown that the RC time constant for each pixel can be increased by up to at least a factor of five (5) by virtue of this structure.
The present invention represents a further improvement to the displays disclosed in U.S. Pat. No. 4,589,733 and in aforementioned U.S. Pat. application Ser. No. 639,001. By virtue of the present invention, even larger auxiliary capacitance can be provided in parallel with the pixel capacitance in such displays.